Gad, I hate to sound like a safety Nazi, but have you considered the possibility of spraying flesh and blood on your face and project with a powerful rotary tool having a high speed fan with exposed, sharp-cornered blades very close to the hand grip tapered toward the fan? I think the Dremel shroud is not so much for fan ducting as for a modicum of safety. Better to experience a friction burn than a bloody wound. Consider also the possibility that the initial contact with the fan would drag the hand into closer proximity to the blades as, does a circular saw, rather than thrusting it away. Of course, I realize that the saw typically has a much more aggressive hook angle than the fan, but I would not like to be the one to experiment. And the fan IS drawing from the rear and blowing out the front.

I like the fan concept, but perhaps a little more effort in fabrication would be worthwhile. Like starting fabrication by drilling a series of holes an appropriate angle just inside the periphery of the disk and Dremelling(?) them out to an aerodynamic shape, leaving a smooth shroud around the blade tips.